It is possible to use the sonar in passive mode, i.e. without its transmission antenna, or in active mode with its transmission antenna formed by the towfish and its receiving antenna. In order to ensure these two operating modes, the towfish is fixed and connected removably to the cable. When the towfish is in place on the cable, it is suspended from the cable such that its center of gravity is situated under the axis of the cable. The towfish comprises a body and one or two arms. The free end of each arm is coupled to the cable from above the cable in order to allow the cable to be guided through the fairlead.
The cable generally comprises a core formed of electrical and/or optical conductors for transmitting energy and information between the equipment of the sonar that are situated on board the ship and the antennas. The core of the cable is generally covered with a strand of metal wires that ensure the mechanical integrity of the cable. The make-up of the cable imposes a minimum radius of curvature thereon. Below this radius, inadmissible mechanical stresses arise and cause the constituents of the cable to deteriorate. The winch fixed to the deck of the ship has a reel on which the cable can be hauled in when the sonar is inactive and when the antennas are stowed on board the ship. The diameter of the reel makes it possible to ensure that the hauled-in elements are not curved at a radius smaller than the minimum radius of curvature.
When the towed elements are in the sea, the cable is guided by the fairlead, which makes it possible to safeguard its effective radius of curvature. The fairlead forms the last element for guiding the cable with respect to the ship before the cable drops into the water. The fairlead comprises a frame fixed to the deck of the ship and a channel in which the cable slides. The channel has an upwardly open section such that the cable is held in the channel by gravity. When the sea is heavy or while the ship is being maneuvered, the cable can escape from the channel, the fairlead then no longer fulfilling its guiding role. In order to prevent the cable from escaping from the channel, it is desirable to close at least a section of the channel. However, the fact of closing the channel prevents the arms of the towfish from passing through the fairlead.
The applicant has attempted to internally produce a fairlead having a closed section that an operator can open manually to allow the arms of the towfish through. The position of the fairlead behind the ship, or partially overhanging the transom of the ship makes the opening and closing operation tricky or even dangerous under difficult navigation conditions. It would be conceivable to remote-control the opening and closing of the fairlead, but this would be complicated to implement. Furthermore, the towing device already requires an operator manipulating the winch. If this operator had to move around in order to open the fairlead, this would entail the risk of the fairlead being left open for an excessively long time. A second operator could manipulate the fairlead, but this would generate a higher operating cost for the towing device.
The invention provides a solution to this problem by proposing a fairlead with a closed channel that can open automatically during the passage of the towfish.